The separation of particulates and macromolecules from a fluid by micro-and ultra-filtration using polymeric membranes has found numerous applications in the analytical, pharmaceutical, electronic, and food industries. Traditionally, porous membranes of cellulose derivatives have been utilized as hydrophilic membrane media for aqueous filtration. However, these membranes have poor chemical resistance, and are also lacking in thermal and gamma sterilizability, which limits their application in practical service.
To meet the requirements for chemical filtration, porous membranes such as polypropylene and polytetrafluoroethylene membranes were therefore developed. These membranes generally have excellent chemical and solvent resistance. However, one drawback associated with these membranes is their hydrophobicity. For practical aqueous filtration, these hydrophobic membranes must be either pre-wetted with a low surface tension liquid such as alcohols, prior to use, or pre-treated with hydrophilic chemicals to render the membrane hydrophilic as described in U.S. Pat. Nos. 4,578,414; 4,501,785 and 4,113,912 prior to use. Upon use of these treated membranes, however, there is a risk that the wetting agents may be leached from the membrane by the solution being filtered. Such leaching in turn may result in contamination of the filtrate.
To impart permanent hydrophilicity to a hydrophobic porous membrane, a hydrophilic monomer can be chemically grafted to the hydrophobic membrane substrate by plasma treatment. The plasma polymerization is generally achieved by activating the membrane surface using argon or hydrogen plasma, followed by exposing the activated membrane to the vapor of free radical polymerizable monomers such as acrylic acid. Typical examples of plasma treatment of membrane can be found in U.S. Pat. No. 4,815,132 and Japanese Patents 59/045,528, 61/152,700 and 56/090,838. In practice, the plasma treatment may fail to produce uniform membrane hydrophilicity due to the low penetration of plasma gas and insufficient access of the hydrophilic monomer to the interior of membrane. Large scale use of the technique is not feasible because of the high vacuum requirement during the treatment.
An alternative to the plasma treatment is to utilize a radiation ray treatment which possesses a high penetrating force such as UV, electron beam or gamma ray treatment. However, the hydrophobic membrane will generally suffer from loss of mechanical strength and other desirable membrane properties after exposure to such high energy irradiation. In addition, these high energy irradiation processes entail unsolved problems regarding oxygen sensitivity and mass production feasibility since the active free-radical sites on the membrane substrate generated by the radiation ray are very susceptible to oxygen scavenge.